U.S. Pat. No. 6,466,036 B1 discloses an art related to a capacitive touch sensor that includes a charge transfer capacitance measurement circuit. In this capacitive touch sensor, when a sense plate is touched by a finger, an electrostatic capacitance is generated between an electrode, which is disposed in an internal portion of the sense plate, and the finger. Then, the electrostatic capacitance is charged to sense the touch of the finger by detecting an amount of an electric charge accumulated between the electrode and the finger.
U.S. Pat. No. 7,312,616 B2 discloses a method of detecting an electrostatic capacitance. In U.S. Pat. No. 7,312,616 B2, the electrostatic capacitance is detected by a successive approximate capacitance measurement circuit. Specifically, a storage capacitor is pre-charged to a predetermined electric potential. When a sensor is touched by a finger, an electrostatic capacitance is generated between an electrode, which is disposed in an internal portion of the sensor, and the finger. The electrostatic capacitance is repeatedly charged and discharged at a predetermined time period so that a predetermined charge current is approximately output from a current source and the storage capacitor.
In U.S. Pat. No. 7,312,616 B2, with above-described configuration, the electric potential of the storage capacitor decreases, and a charge time to reset the electric potential of the storage capacitor to the predetermined electric potential changes. The charge time changes depending on the charge current. Further, the charge current changes depending on the electrostatic capacitance generated between the electrode and the finger. Thus, a touch on the sensor is detected by measuring a change of the charge time.
As described above, the electrostatic capacitance is detected by repeatedly performing a charge and a discharge to the electrostatic capacitance. Thus, a switching circuit is necessary to repeatedly perform the charge and the discharge. When the switching circuit controls switching elements to repeatedly open and close to perform the charge and the discharge, a radiation noise is generated from the switching circuit. Since, the radiation noise is generated during a switching operation of the switching circuit, the radiation noise is also referred to as a switching noise. A frequency of the switching circuit to perform the charge and the discharge generally has a range of several dozen kilohertz (kHz) to several hundred kHz.
In order to reduce the radiation noise, JP 2010-272991 A discloses a method of reducing the radiation noise by adding a detection electrode. Further, JP 2009-177677 A discloses a method of reducing the radiation noise using a spread-spectrum technique. Specifically, a peak radiation noise level is reduced by randomly changing a frequency of a pulse signal to spread a spectrum of the radiation noise.
A noise reduction effect of the spread-spectrum technique depends on a band width of a spread frequency band. Thus, the spread-spectrum technique has a limited noise reduction effect. Further, when the frequency is spread, a harmonic noise is distributed in a wide range. Thus, spread-spectrum technique fails to reduce the radiation noise radically.